1. Field of the Invention
The present invention relates to a technique of generating an analysis model for analysis of an object from a three-dimensional geometric model obtained by modeling the three-dimensional geometry of the object.
2. Description of the Related Art
Recently, a three-dimensional CAD (computer aided design) system has been widely used in designing an apparatus including a plurality of parts. The three-dimensional CAD is used in various devices such as an integrated circuit, a machine, a vehicle, a building, etc. With the improvement of computer performance, a more complicated three-dimensional geometric model can be generated by the three-dimensional CAD system. In the following descriptions, the three-dimensional geometric model is simply referred to as a “geometric model”. At present, a computer having the performance, which is sufficient to process a geometric model as detailed as the form of an ultimate product, has become widespread. Therefore, there are not a few cases where design models are as detailed as practical product models.
On the other hand, a mechanical analysis of the distortion caused by applying external force on an apparatus and the strength of the apparatus can also be generally performed by a computer.
The performance of a number of computers being widely used is sufficient to process a detailed specific geometric model, but insufficient to perform a numerical analysis using the detailed specific geometric model as an analysis model. Therefore, it is common that an analysis model for analysis is generated in addition to a geometric model for design, and a numerical analysis is performed in a finite-element method using the analysis model.
For example, there is an apparatus having a locker unit structure including a metal girder, top plate, and floor plate. The locker unit structure is a skeleton framework, and stores various parts in the internal space. The ultimate product can store various parts, but it is important to mechanically analyze the locker unit structure.
In the locker unit structure, it is common that fastening parts such as a screw, a bolt, a rivet, welding, etc. are used in fastening sheet-metal parts such as a girder, a top plate, a floor plate, etc. Recently, including the fastening parts, a specific geometric model defined for the detailed geometry of a metal sheet is often generated using the three-dimensional CAD system. However, such a geometric model is so specific that a large volume of computer resources are required in analysis when it is used as an analysis model.
For example, when screws or bolts are used in a fastening operation, there is naturally a screw hole. When an analyzing process is performed in the finite-element method, a plurality of nodes of a mesh is located along the outline of the hole, and the nodes have to be coupled to other nodes of the mesh. As a result, there is an increasing number of nodes directly or indirectly, and a large volume of computer resources are required in analysis. Therefore, it is impractical to utilize a detailed specific geometric model as an analysis model with the performance of the currently marketed computer taken into account.
Therefore, it is common to generate an analysis model by simplifying a specific geometric model. To simplify the model, only a basic structure is first generated with the assembly parts omitted as much as possible. Especially relating to a thin plate such as sheet-metal parts, it is common that a solid model generated by the three-dimensional CAD system is replaced with a shell model as a face model. In the shell model, the fastening portion by a screw, a bolt, a rivet, etc. is replaced with point-to-point connection by connecting points using, for example, a beam element.
A shell model is briefly described below with reference to FIGS. 1A and 1B.
FIG. 1A is a perspective view showing two plates 121 and 122 fastened by bolt fastening 123 and 124. FIG. 1A shows the geometric model generated by the three-dimensional CAD system. As shown in FIG. 1A, the geometric model generated by the three-dimensional CAD system is a solid model, and each of the plates 121 and 122 is shown as a geometry having a thickness.
On the other hand, FIG. 1B is a shell model corresponding to FIG. 1A. In FIGS. 1A and 1B, the plate 121 corresponds to a face 125, the plate 122 corresponds to a face 126, the bolt fastening 123 corresponds to point-to-point connection 127, and the bolt fastening 124 corresponds to point-to-point connection 128.
As shown in FIG. 1B, the plates 121 and 122 of the three-dimensional geometries having thicknesses are replaced with the faces 125 and 126 as planes compressed in the thickness direction in the shell model. The faces 125 and 126 are called medial surfaces. The fastening by a bolt, a rivet, welding, etc. is replaced with point-to-point connection by connecting a point on the face 125 to a point on the face 126. In the example shown in FIG. 1B, the two points of bolt fastening 123 and 124 are replaced with the two point of point-to-point connection 127 and 128. However, for example, the fastening by an adhesive can be replaced with n points of point-to-point connection.
An example of using a shell model is described in the patent documents 1 and 2.
The analysis model generation apparatus (analytic model preparing apparatus) described in the patent document 1 automatically retrieves a fastening portion (a joint portion) from a geometric model. In retrieving, the position of the fastening and the type of fastening such as a screw, a bolt, a rivet, welding, etc. is determined. Then, the fastening portion obtained as a result of the retrieval is highlighted, and a user is allowed to confirm whether or not the determination is correct. Thus, the analysis model generation apparatus refers to the joint model preparing object database and models the fastening portion in a shell model for the fastening portion whose position and type have been determined.
The patent document 2 describes a method of stacking a single layer model for each layer and analyzing the model to analyze a multilayer printed circuit board.    [Patent Document 1] Japanese Published Patent Application No. 2001-265836    [Patent Document 2] Japanese Published Patent Application No. 2006-91939
The following steps are required to generate a shell model for analysis from a geometric model.
(1) A step of extracting a basic structure from a solid model having a large volume of information. For example, a step of extracting the plates 121 and 122 as a basic structure from a geometric model shown in FIG. 1A.
(2) A step of replacing a solid model with a shell model. For example, a step of replacing the plates 121 and 122 with the faces 125 and 126.
(3) A step of appropriately replacing a fastening portion. For example, a step of replacing the bolt fastening 123 and 124 with two points of the point-to-point connection 127 and 128.
These steps often require manual operations. Especially, the step (3) above requires determination by a person. However, since not a few apparatuses include 200 to 300 points of fastening portions, there are an enormously large number of operation steps. In addition, it is very difficult to correctly express the rigidity (i.e. stiffness) of the fastening portion, and there is no established method of modeling a fastening portion. Accordingly, there is uneven analysis accuracy.
Although a method for reducing the influence of replacing operations has been studied, the replacement described in (2) above has an influence on the analysis accuracy.
On the other hand, if a geometric model itself is used as is as an analysis model to avoid the above-mentioned problems, a mesh division is performed on the geometric model. Therefore, the number of meshes and the number of nodes configuring the meshes become large, and the analysis requires a large volume of computer resources.
This problem is first caused by a large number of small meshes required for the bolts and nuts used for the bolt fastening 123 and 124 shown in FIG. 1A.
Second, the problem is caused by some nodes of meshes to be set along the outline of the holes through which the bolts penetrate the plates 121 and 122. Then, the meshes including the nodes are radially set on the periphery of the hole. If there is no hole, the plates 121 and 122 are simply rectangular parallelepiped. Therefore, they can be appropriately modeled by coarse meshes, and the number of nodes on the top and bottom surfaces of the plates 121 and 122 is low. However, the diameter of the hole for a bolt is generally much smaller as compared with the sizes of the plates 121 and 122, and there are a plurality of nodes along the periphery of the small hole. Therefore, if nodes are set along the outline of a hole, then meshes are closely set radially on the periphery of the hole, and the closely arranged meshes directly or indirectly increase the number of necessary nodes on the top and bottom surfaces of the plates 121 and 122.